One application for the present invention is the optical examination of the gasket being formed on a container closure which is presented to an inspection station, and then moved, e.g. rotated, at the inspection station to allow a single scanning head to scan a gasket strip, e.g. a circular track, on the container closure, and then advanced from the inspection station and followed by a further such closure. The optical examination of the container closure may, for example, be for the purposes of inspecting the completeness of a sealing gasket formed in the container closure which may, for example, be a bottle or jar closure, a can end, or a drum or pail cover.
However, the invention is not to be limited to use with circular workpieces because it is well known for container closures such as can ends to have an other than circular form (for example rectangular) and to be provided with a gasket over a non-circular peripheral portion adjacent its perimeter.
It is known to inspect circular closures optically for the purposes of confirming the presence and/or continuity of an annular gasket formed near the perimeter of the closure. The closures, for example can ends or bottle or jar caps, are presented in rapid succession to the inspection station, which may also be the lining station, and are to be examined once they are in position on a rotary chuck which holds each can end while it is being rotated to bring the entire length of the peripheral gasket region of the closure past an optical scanning head. However, it is important to define when the scanning operation is to be activated and to cease, because during scanning it is intended that the container closure or other workpiece be held so that its zone passing the optical scanning head presents a constant optically observed quality, for example a constant reflectance to indicate the presence of a gasket material. In the case of gasketing can ends, an ungasketed can end or a portion on which the gasket may be discontinuous, will have a high reflectance value indicating the lack of reflection-attenuating gasketing material whereas the presence of the gasket will give rise to a reflectance value of a predetermined range less than that encountered with an ungasketed can end but above a signal value which may be encountered in the case of a fault in the gasket (for example the presence of a dark impurity particle) or a discontinuity of the can end.
It is known in such inspection equipment to initiate the scanning operation once the container closure is in place on the rotatable chuck, and to define a scanning window in terms of a time delay terminating when it is expected that the scanning rotation of the container closure will have been completed, and before onward advance of the container closure from the inspection station begins.
We have found that the traditional method of counting the time elapsed from the start of the scanning operation to define the end of the "scanning window" has the disadvantage that the timing is specific to the design of the closure lining apparatus with which the inspection equipment is used, and when the machine speed is altered it is necessary to compensate by changing the count value of the clock which defines the width of the "scanning window". Furthermore, it is also frequently the case that the instant of the start of the "scanning window" is in need of compensation when the machine speed is changed.